(defclass a () ())

(eval-when (:compile-toplevel)
  (defmacro test (&environment env)
    (print (find-class 'a NIL env)))
  (test))

;; (compile-file this)
; compiling file "/home/linus/a.lisp" (written 30 OCT 2018 04:39:40 PM):
; compiling (DEFCLASS A ...)

; file: /home/linus/a.lisp
; in: EVAL-WHEN (:COMPILE-TOPLEVEL)
;     (EVAL-WHEN (:COMPILE-TOPLEVEL)
;       (DEFMACRO TEST (&ENVIRONMENT ENV) (PRINT (FIND-CLASS 'A NIL ENV)))
;       (TEST))
; 
; caught STYLE-WARNING:
;   The variable ENV is defined but never used.
NIL

;;; But according to http://www.lispworks.com/documentation/HyperSpec/Body/m_defcla.htm :
;;;
;;; "The compiler must make the class definition available to be returned by find-class when its environment argument is a value received as the environment parameter of a macro."